Fallout is the residual radiation hazard from a nuclear explosion and is named from the fact that it "falls out" of the atmosphere in to which it is spread during the explosion. It commonly refers to the radioactive dust created when a nuclear weapon explodes, although it can also refer to nuclear accidents. This radioactive dust is a kind of radioactive contamination.
Source of fallout
A nuclear explosion vaporizes any material within the fireball, including the ground if it is nearby and this is combined with residual ionizing radiation to produce fallout. The sources of this residual ionizing radiation are:
- Fission Products. These are intermediate weight isotopes which are formed when a heavy uranium or plutonium nucleus is split in a fission reaction. There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. Some are very short, i.e., fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta and gamma radiation. Approximately 60 g of fission products are formed per kiloton of yield. The estimated activity of this quantity of fission products 1 minute after detonation is 1.1 ZBq, equal to that of 30 Gg of radium, in equilibrium with its decay products.
- Unfissioned Nuclear Material. Nuclear weapons are relatively inefficient in their use of fissionable material, and much of the uranium and plutonium is dispersed by the explosion without undergoing fission. Such unfissioned nuclear material decays by the emission of alpha particles and is of relatively minor importance.
- Neutron-Induced Activity. If atomic nuclei capture neutrons when exposed to a flux of neutron radiation, they will, as a rule, become radioactive (neutron-induced activity) and then decay by emission of beta and gamma radiation over an extended period of time. Neutrons emitted as part of the initial nuclear radiation will cause activation of the weapon residues. In addition, atoms of environmental material, such as soil, air, and water, may be activated, depending on their composition and distance from the burst. For example, a small area around ground zero may become hazardous as a result of exposure of the minerals in the soil to initial neutron radiation. This is due principally to neutron capture by sodium (Na), manganese, aluminum, and silicon in the soil. This is a negligible hazard because of the limited area involved.
Types of fallout
After an air burst the fission products, unfissioned nuclear material, and weapon residues which have been vaporized by the heat of the fireball will condense into a fine suspension of very small particles 10 nm to 20 µm in diameter. These particles may be quickly drawn up into the stratosphere, particularly if the explosive yield exceeds 10 kt. They will then be dispersed by atmospheric winds and will gradually settle to the earth's surface after weeks, months, and even years as worldwide fallout.
The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and caesium-137, in the body as a result of ingestion of foods incorporating these radioactive materials. This hazard is much less serious than those which are associated with local fallout and, therefore, is not discussed at length here. Local fallout is of much greater immediate operational concern.
This type of fallout is featured in the novel On the Beach by British author Nevil Shute.
In a land or water surface burst, large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses, with fission products and other radiocontaminants that have become neutron-activated.
There will be large amounts of particles of less than 100 nm to several millimeters in diameter generated in a surface burst in addition to the very fine particles which contribute to worldwide fallout. The larger particles will not rise into the stratosphere and consequently will settle to earth within about 24 hours as local fallout.
Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations. The ground track of fallout from an explosion is a long, thin fuzzy ellipse downwind of the explosion. It may be hundreds of kilometers long, and up to 50 km (30 miles) wide from a single explosion.
Whenever individuals remain in a radiologically contaminated area, such contamination will lead to an immediate external radiation exposure as well as a possible later internal hazard due to inhalation and ingestion of radiocontaminants.
Factors affecting fallout
In cases of water surface (and shallow underwater) bursts, the particles tend to be rather lighter and smaller and so produce less local fallout but will extend over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding affect causing local rainout and areas of high local fallout. Fallout from seawater is unusually dangerous because it is difficult to remove by washing.
For subsurface bursts, there is an additional phenomenon present called "base surge." The base surge is a cloud that rolls outward from the bottom of the column produced by a subsurface explosion. For underwater bursts the visible surge is, in effect, a cloud of liquid (water) droplets with the property of flowing almost as if it were a homogeneous fluid. After the water evaporates, an invisible base surge of small radioactive particles may persist.
For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a fluid. A soil earth medium favors base surge formation in an underground burst.
Meteorological conditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to distribute fallout over large areas. For example, as a result of a surface burst of a 15 Mt thermonuclear device at Bikini Atoll on March 1, 1954, a roughly cigar-shaped area of the Pacific extending over 500 km downwind and varying in width to a maximum of 100 km was severely contaminated.
Snow and rain, especially if they come from considerable heights, will accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination just downwind of a nuclear blast may be formed.
Effects of fallout
A wide range of biological changes may follow the irradiation of animals. These vary from rapid death following high doses of penetrating whole-body radiation, to essentially normal lives for a variable period of time until the development of delayed radiation effects, in a portion of the exposed population, following low dose exposures.
Median Lethal Dose (LD50): When comparing the effects of various types or circumstances, that dose which is lethal to 50% of a given population is a very useful parameter. The term is usually defined for a specific time, being limited, generally, to studies of acute lethality. The common time periods used are 30 days or less for most small laboratory animals and to 60 days for large animals and humans. It should be understood that the LD50 assumes that the individuals did not receive other injuries or medical treatment.
Initial radiation from fallout can exceed 300 gray per hour (Gy/h) immediately downwind of a ground burst . A cumulative dose of 4.5 Gy is fatal to half of a population of humans. There have been no documented cases of survival beyond 6 Gy. Most people become ill after an exposure to 1 Gy or more. The fetuses of pregnant women are vulnerable and may miscarry, especially in the first trimester. Human biology resists mutation from large radiation exposure: grossly mutated fetuses usually miscarry. Civilian dose rates in peace-time range from 30 to 100 µGy/y.
Fallout radiation falls off ('decays') exponentially (quickly) with time. Most areas become safe for travel and decontamination after three to five weeks.
The most dangerous emissions from fallout are gamma rays, which travel in straight lines, like ordinary light. The fallout particles emit the invisible, deadly gamma rays in the same way that a light bulb emits light. Gamma rays are invisible, and cannot be seen, smelt, or felt. Special equipment is required to detect and measure gamma rays.
For yields of up to 10 kt, initial nuclear radiation is the dominant casualty producer on the battlefield. Humans receiving an acute incapacitation dose (30 Gy) will become performance degraded almost immediately and ineffective within several hours. However, they will not die until 5 to 6 days after exposure assuming they do not receive any other injuries which make them more susceptible to the radiation dose. Individuals receiving less than a total of 1.5 Gy will remain effective. Between those two extremes, people receiving doses greater than 1.5 Gy will become degraded; some will eventually die.
A dose of 5.3 Gy to 8.3 Gy is considered lethal but not immediately incapacitating. Personnel exposed to this amount of radiation will become performance degraded within 2 to 3 hours, depending on how physically demanding the tasks they must perform are, and will remain in this degraded state at least 2 days. However, at that point they will experience a recovery period and be effective at performing non-demanding tasks for about 6 days, after which they will relapse into a degraded state of performance and remain so for about 4 weeks. At this time they will begin exhibiting radiation symptoms of sufficient severity to render them totally ineffective. Death follows at approximately 6 weeks after exposure.
Late or delayed effects of radiation occur following a wide range of doses and dose rates. Delayed effects may appear months to years after irradiation and include a wide variety of effects involving almost all tissues or organs. Some of the possible delayed consequences of radiation injury are life shortening, carcinogenesis, cataract formation, chronic radiodermatitis, decreased fertility, and genetic mutations.
Tactical military considerations
Blast and thermal injuries, due to the use of nuclear weapons for military action, in many cases will far outnumber radiation injuries. However, radiation effects are considerably more complex and varied than are blast or thermal effects and are subject to considerable misunderstanding.
The closer to ground an atomic bomb is detonated, the more dust and debris is thrown into the air, resulting in greater amounts of local fallout. From a tactical standpoint, this has the disadvantage of hindering any occupation efforts until the fallout clears, but more directly, the impact with the ground severely limits the destructive force of the bomb. For these reasons, ground bursts are not usually considered tactically advantageous, with the exception of hardened underground targets such as missile silos or command centers such as Cheyenne Mountain. "Salting" enemy territory with a fallout-heavy atomic burst could be used to deny enemy access to a contaminated area but such use is generally not considered an ethical military action.
Prevention and Cleanup
In severe cases of fallout contamination, lethal doses of external radiation may be incurred if protective or evasive measures are not undertaken. A fallout shelter is designed to shield its occupants from radiation. However, some radioactive contamination will probably remain when the inhabitants eventually emerge.
Gamma rays do not contaminate people or objects. Fallout particles contaminate people or objects, and since they resemble sand, they can be brushed off, or washed off. The radioactive fog from seawater is a notable exception, being very difficult to wash off. The particles should be removed from the shelter, or shielded. Emergency drinking water can be adequately cleaned by filtering contaminated water through more than 25 cm (10 in) of dirt. Food in sealed packages is not poisoned by fallout. Stored grain and exposed fruit can be cleaned and peeled. Vehicles are usually washed down with fire-hoses, into drains with removable filters, or deep trenches. Ground is usually decontaminated by bulldozing the fallout into deep, narrow trenches, and then back-filling the trenches.
The fallout residue can be used to analyze the source and nature of the weapon used. Materials used in the weapon will have distinct signature, which with proper analysis could reveal where or by whom the weapon was made.
- http://www.cddc.vt.edu/host/atomic/nukeffct/ - For videos and more on the effects of a thermonuclear device.
- Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons (third edition), U.S. Government Printing Office, 1977. (Available Online)
NATO Handbook on the Medical Aspects of NBC Defensive Operations (Part I - Nuclear), Departments of the Army, Navy, and Air Force, Washington, D.C., 1996. (Available Online)
- Smyth, H. DeW., Atomic Energy for Military Purposes, Princeton University Press, 1945. (Smyth Report)
- The Effects of Nuclear War, Office of Technology Assessment (May 1979)
Last updated: 10-16-2005 14:34:23